Some electronic switching power supplies must provide for normal operation during transient interruptions in a primary power source. In certain military applications, for example, the required holdup period is fifty milliseconds. Such power supplies typically employ a capacitor bank on the output side of the transformer to provide the auxiliary power required during the holdup period.
Vesce et al U.S. Pat. No. 4,593 213 is illustrative of the prior art. During normal operation a capacitor bank 148 is charged from a secondary winding 224. An interruption in the primary input voltage is sensed by a monitoring circuit 164 which enables a holdup enable circuit 160. The enable circuit 160 in turn enables a holdup transfer switch 152, which permits current flow from the capacitor bank 148 to the primary input line 110. Current flow from the primary input line 110 through the primary winding 200 is then modulated in a conventional manner using a FET 202 and switching regulator 114.
A shortcoming of the method described in the above-cited patent is that it limits energy transfer efficiency. Efficiency is limited due to series resistance in the loop between the capacitor bank 148 and the primary winding.
An objective of the present invention is to provide a switching power supply which is operable to provide substantially uninterrupted power during a holdup period of at least fifty milliseconds while achieving an energy transfer efficiency of at least ninety percent.